1. Field of the Invention
The present invention relates to a control system of servo motors which controls a machine tool or other machine. The present invention particularly relates to a control system of servo motors wherein a plurality of axes which are driven by servo motors are coordinated to improve a processing precision when processing a closed curve or polygon.
2. Description of the Related Art
In general, in a drilling operation which uses a machining center or other machine tool, a boring operation which uses a dedicated tool is performed for improving the precision. The boring tool which is used in the boring operation has to be selected in accordance with the diameter of the hole being bored. Therefore, when boring holes of a plurality of different diameters, a plurality of boring tools become necessary.
Usually, in a machining center which is equipped with a numerical control device (CNC), an automatic tool changer (ATC) is provided. The ATC can be equipped with a plurality of tools, so the tools can be automatically changed in accordance with the operation. However, there is a limit to the number of tools which can be mounted at an ATC. For this reason, when an operation arises which is performed by a tool which is not stored at the ATC, time is required for changing the tool. There are therefore issues such as deterioration of the productivity.
As opposed to this, there is the method of replacing a boring operation which bores a hole with a milling operation. As one of the methods for the alternative operation of a milling operation, there is a helical machining operation. A helical machining operation is a machining operation using an end mill suitable for the diameter of the hole to be bored so as to repeat an arc motion in a helical manner so as to bore the hole. In a helical machining operation, in general, the arc motion is performed by the coordinated drive operation of a table which carries a workpiece by servo motors in the X-axis and the Y-axis directions.
When using a helical machining operation to bore a hole, even when boring a plurality of holes with different diameters, there is the advantage that a single end mill can handle the operation. On the other hand, a helical machining operation is accompanied with arc motion of the table, so if increasing the speed, due to the delayed response of the X-axis and Y-axis servo motors which drive the table or lost motion of the machinery (backlash or torsion), quadrant glitch (delay at time of reversal of drive axis) occurs and the precision is liable to deteriorate.
As a method for countering this deterioration of precision, the method of utilizing the fact that in a helical machining operation, the end mill repeats arc motion and of applying learning control to the control of the servo motors of the X-axis and Y-axis drive axes is the most effective. In particular, it is effective to apply angle synchronization type learning control enabling fluctuations of the processing speed to be handled as described in the servo motor drive control system which is disclosed in Japanese Patent No. 4043996, but in this case, angular information serving as the basis for the period of the learning control becomes necessary. The reference angle has to made to monotonously increase or monotonously decrease, but in the case of arc motion of an end mill by the two X-axis and Y-axis, there is no information (signal) which can be utilized as this reference angle.
To solve this problem, Japanese Patent No. 4980453B2 discloses a servo control system which cumulatively adds the X-axis or Y-axis command values or absolute values of the feedback values to thereby create information (signal) at the reference angle.
However, the servo control system which is disclosed in Japanese Patent No. 4980453 has the issue of being unable to handle the case where the diameter of the arc slowly changes. This issue arises since learning control requires the period of learning (for example 360 degrees etc.) as projected information in order to secure a memory corresponding to the period of repetition of the command value, but when preparing a signal corresponding to the reference angle from the command values or feedback values, the angle which the signal indicates is not the angle from the center, but the amounts of movement of the X-axis and Y-axis, so if the diameter of the circle changes, the amounts of movement change. As a result, in the servo control system which is disclosed in Japanese Patent No. 4980453, the period of the projected information and the actual amount of movement no longer match and correct learning control becomes no longer possible.
For example, in a helical machining operation, when an end mill engages in circular motion such as illustrated in FIG. 1A, it is possible (easy) to calculate the amount of movement of the end mill corresponding to the learning period from the diameter of the circle “c”. However, in a helical machining operation, when an end mill engages in motion such as the free closed curve L not a simple circle such as illustrated in FIG. 1B, it is not easy to calculate the amount of movement of the end mill.